Re: [PATCH v7 01/14] xen/common: add cache coloring common code

From: Jan Beulich <jbeulich@suse.com>
To: Carlo Nonato <carlo.nonato@minervasys.tech>
Cc: Andrea Bastoni <andrea.bastoni@minervasys.tech>,
	Andrew Cooper <andrew.cooper3@citrix.com>,
	George Dunlap <george.dunlap@citrix.com>,
	Julien Grall <julien@xen.org>,
	Stefano Stabellini <sstabellini@kernel.org>, Wei Liu <wl@xen.org>,
	Marco Solieri <marco.solieri@minervasys.tech>,
	xen-devel@lists.xenproject.org
Subject: Re: [PATCH v7 01/14] xen/common: add cache coloring common code
Date: Fri, 22 Mar 2024 08:25:12 +0100	[thread overview]
Message-ID: <cbcbeace-7d19-4e73-a12b-d2bcbc3b51f3@suse.com> (raw)
In-Reply-To: <CAG+AhRUJiUrnD6Ovvvy0_Naa-RnOWuYPDZqPJx2-zd7eUa828Q@mail.gmail.com>

On 21.03.2024 18:22, Carlo Nonato wrote:
> Hi Jan,
> 
> On Thu, Mar 21, 2024 at 4:53 PM Jan Beulich <jbeulich@suse.com> wrote:
>>
>> On 21.03.2024 16:03, Carlo Nonato wrote:
>>> On Tue, Mar 19, 2024 at 3:58 PM Jan Beulich <jbeulich@suse.com> wrote:
>>>> On 15.03.2024 11:58, Carlo Nonato wrote:
>>>>> --- a/docs/misc/xen-command-line.pandoc
>>>>> +++ b/docs/misc/xen-command-line.pandoc
>>>>> @@ -1706,6 +1706,43 @@ This option is intended for debugging purposes only.  Enable MSR_DEBUGCTL.LBR
>>>>>  in hypervisor context to be able to dump the Last Interrupt/Exception To/From
>>>>>  record with other registers.
>>>>>
>>>>> +### llc-coloring
>>>>> +> `= <boolean>`
>>>>> +
>>>>> +> Default: `false`
>>>>> +
>>>>> +Flag to enable or disable LLC coloring support at runtime. This option is
>>>>> +available only when `CONFIG_LLC_COLORING` is enabled. See the general
>>>>> +cache coloring documentation for more info.
>>>>> +
>>>>> +### llc-nr-ways
>>>>> +> `= <integer>`
>>>>> +
>>>>> +> Default: `Obtained from hardware`
>>>>> +
>>>>> +Specify the number of ways of the Last Level Cache. This option is available
>>>>> +only when `CONFIG_LLC_COLORING` is enabled. LLC size and number of ways are used
>>>>> +to find the number of supported cache colors. By default the value is
>>>>> +automatically computed by probing the hardware, but in case of specific needs,
>>>>> +it can be manually set. Those include failing probing and debugging/testing
>>>>> +purposes so that it's possibile to emulate platforms with different number of
>>>>> +supported colors. If set, also "llc-size" must be set, otherwise the default
>>>>> +will be used.
>>>>> +
>>>>> +### llc-size
>>>>> +> `= <size>`
>>>>> +
>>>>> +> Default: `Obtained from hardware`
>>>>> +
>>>>> +Specify the size of the Last Level Cache. This option is available only when
>>>>> +`CONFIG_LLC_COLORING` is enabled. LLC size and number of ways are used to find
>>>>> +the number of supported cache colors. By default the value is automatically
>>>>> +computed by probing the hardware, but in case of specific needs, it can be
>>>>> +manually set. Those include failing probing and debugging/testing purposes so
>>>>> +that it's possibile to emulate platforms with different number of supported
>>>>> +colors. If set, also "llc-nr-ways" must be set, otherwise the default will be
>>>>> +used.
>>>>
>>>> Wouldn't it make sense to infer "llc-coloring" when both of the latter options
>>>> were supplied?
>>>
>>> To me it looks a bit strange that specifying some attributes of the cache
>>> automatically enables cache coloring. Also it would require some changes in
>>> how to express the auto-probing for such attributes.
>>
>> Whereas to me it looks strange that, when having llc-size and llc-nr-ways
>> provided, I'd need to add a 3rd option. What purpose other than enabling
>> coloring could there be when specifying those parameters?
> 
> Ok, I probably misunderstood you. You mean just to assume llc-coloring=on
> when both llc-size and llc-nr-ways are present and not to remove
> llc-coloring completely, right?

Yes. The common thing, after all, will be to just have llc-coloring on the
command line.

Jan